Common Mistakes🔖

These are the mistakes almost everyone makes in Week 3. Most of them fail silently, which makes them particularly frustrating.

Missing name= on a form input🔖

<!-- WRONG — Flask can't find this field -->
<input type="text" id="title">
 
<!-- RIGHT -->
<input type="text" name="title" id="title">

request.form is keyed by name, not id. Without name, the field doesn't exist in Flask's view. You'll get a KeyError when Python tries to read it, or an empty value if you used .get(). The form will look fine — the bug is invisible until submission.

Missing method="POST" on the form🔖

<!-- WRONG — defaults to GET, form data doesn't reach request.form -->
<form action="/input">
 
<!-- RIGHT -->
<form action="/input" method="POST">

Without method="POST", the browser sends a GET request. Form data goes into the URL as query parameters, not the request body. request.form will be empty. The route's GET handler runs instead of the POST handler, and the form just re-renders itself.

Missing methods=["GET", "POST"] on the route🔖

# WRONG — Flask only accepts GET by default
@app.route("/input")
def input_task():
    ...
 
# RIGHT
@app.route("/input", methods=["GET", "POST"])
def input_task():
    ...

Without the methods= list, Flask returns 405 Method Not Allowed when the form is submitted. You'll see the error in the browser and a traceback in the terminal.

Using "w" mode instead of "a" when writing the CSV🔖

# WRONG — deletes all existing rows before writing
with open("tasks.csv", "w", newline="") as f:
 
# RIGHT — appends the new row to the end
with open("tasks.csv", "a", newline="") as f:

"w" (write) truncates the file to zero bytes before writing. Every form submission wipes your entire task history. You won't notice until you've submitted a few tasks, visited /display, and found only one row. Check tasks.csv directly to confirm whether your rows are actually being kept.

Missing newline="" when opening for CSV writing🔖

# WRONG — extra blank lines appear between rows on some systems
with open("tasks.csv", "a") as f:
 
# RIGHT
with open("tasks.csv", "a", newline="") as f:

Without newline="", the csv module's line terminator and Python's own newline handling combine to insert a blank line after every row. DictReader will include these blank rows, and you'll see empty rows appearing in your table.

Rendering a template after POST instead of redirecting🔖

# WRONG — pressing refresh re-submits the form
if request.method == "POST":
    # ... save data ...
    return render_template("display.html", tasks=tasks)
 
# RIGHT — redirect breaks the cycle
if request.method == "POST":
    # ... save data ...
    return redirect(url_for("display"))

If you render directly after POST, the browser's "last request" was a POST. Refreshing the page asks "resend this POST?" and submitting again creates a duplicate row. Use redirect() so the browser's last request becomes a GET.

Missing list= on the project_title input🔖

<!-- WRONG — datalist exists but input isn't connected to it -->
<input type="text" name="project_title" id="project_title">
 
<!-- RIGHT -->
<input type="text" name="project_title" id="project_title" list="project-suggestions">

The list attribute value must match the id of the <datalist> exactly. Without it, the autocomplete suggestions won't appear — but the field still works, it's just not helpful.